High-frequency insertion loss test system including coupled attenuators



Sept. 17, 1963 B. o. wElNscHEL l-:TAL 3,104,354

HIGH-FREQUENCY INSERTION LOSS TEST SYSTEM INCLUDING COUPLED ATTENUATORS2 Sheets-Sheet l Filed Aug. 5l. 1961 INVENTORS 36u/vo O. WE/Nscf/ELGru/urns@ U50/Q65@ 442m@ .n.N.

ATTORNEY Sept. 17, 1963 B. o. wElNscHEL ETAL 3,104,354

HIGH-FREQUENCY INSERTION LOSS TEST SYSTEM INCLUDING COUPLED ATTENUATORS2 Sheets-Sheet 2 Filed Aug. 3l. 1961 LOCAL OSC( LLATQQ COMFIA 2- ISDNHEAD E C 2 U o S DWF AMPL.

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ATTORNEY United States Patent O 3,104,354 HIGH-FREQUENCY NSERTIN LGSSTEST SYS- 'iElt'l NSLUBIG CGUPLED ATTENUATRS Bruno Weinschei, Bethesda,and Gunther U. Sergei', Rockville, Md., assignors to V/einscnelEngineering C0., fue., Kensington, Md., a ce1-poration of Delaware FiledAng. 31, 11561, Ser. No. 136,153 Claims. (El. 324-58) This inventionrelates to high-frequency calibration systems land apparatus, andparticularly to improvements in a system such as described in thecopending application Serial No. 64,766 now Patent No. 3,034,045 ofBruno O. Weinschel for Voltage Ratio Meter for High-FrequencyCalibration Systems. The system to which the invention pertains is alsodescribed in an article by Hedrich et al., Calibration 0f SignalGenerator Output Voltage in the Range of 100 to 1000 Megacycles, IRETransactions on Instrumentation, December, 1958, vol. 1-7, Nos. 3 and 4,pp. 274-279.

In systems of the above type, an LF. signal derived from the RF.generator being tested, and another signal from a standard source at thesame LF. frequency, are fed in interlaced fashion to an LF. amplifier,where, in effect, the signal being tested is compared with the signalfrom the standard source. In the systems previously described, both theLF. frequency and the standard frequency must be exactly alike, first,because the bandwidth of frequency response of fthe LF. amplifier cannotbe made really at, and secondly, because the LF. amplifier responsechanges with level. This is particularly true if the changes in levelare very large, which is a practical requirement in .these systems. Inthe above prior art systems, the LF. differential amplifier has anautomatic gain control (A.G.C.) circuit so arranged that the output,that is, the rectified maximum D.C. output voltage from the lineardetector is always kept constant regardless of Ithe input voltage at theLF. frequency. The sensitivity of the system thus stays constantregardless of the RF. level. This may require the LF. amplifier tochange its gain, in some cases .as much Ias 60 db, since the R.F. signalalso may vary by 60 db. Such a large change requires that the frequencyresponse of the LF. amplifier must remain identical for a 60 db gainrange, if, assuming that both frequencies are not alike the gain forboth signals should stay exactly alike.

According to the present invention, this ditiiculty is avoided by alwaysrunning the LF. amplifier .at a constant LF. input level, at constantgain; in this case, the two frequencies can differ by a small amountwithout doing any harm, because the LF. amplifier is sufficiently stableduring the short period of .the measurement not `to change its frequencyresponse appreciably during that interval.

In the prior art systems above described, the major problem is reallydue to the need for a very large gain change. With constant gain, as inthe present invention, the frequency response will stay sufficientlyconstant for practical purposes.

It is a primary object of the invention to overcome the above diicultyin the prior art `systems by simple means which permit the LF. amplifierto be run at a constant LF. input level and at a constant gain.

The above and other objects are accomplished, according to theinvention, by inserting a variable attenuator between the comparisonhead and the LF. differential amplifier, the variable attenuator beinglinked to the operation of the 30 mc. precision piston attenuator insuch a manner that the -combined attenuation of the two is constant.

The specific nature of the invention as well as other objects andadvantages thereof will clearly appear from a description of a preferredembodiment as shown in the accompanying drawings, in which:

FIG. l is a circuit diagram of the entire measuring system, in blockform;

FIG. 2 is a schematic circuit diagram showing in more detail those partsof the circuit with which the present invention is concerned; and

FIG. 3 is a schematic diagram of a mechanical arrangement for carryingout the invention.

FIG. l is an essentially self-explanatory diagram in block form of .thecircuit. The circuit elements from the signal generator 4 to the 30 mc.comparison head 16 are essentially the same as in the above-referencedprior art circuit. Briefly, the R.F. signal being measured is suppliedon line 10 to mixer 7, where -it is heterodyned with the output of localoscillator 12 to provide sa 30 mc. output on line 13 to the 30 mc.comparison head 16; this head is also supplied with the output from fthe30 mc. standard source 14, through .an adjustable piston iattenuator 17,the two inputs being supplied in interlaced fashion, due to the actionlof modulator 18, which, in effect, alternately switches each input intothe comparison head at a 1 kilocycle rate. The interlaced outputs arefed, according to the prior art disclosures, on line 15 directly tot LF.differential amplifier 23. lIn the prior .art disclosures, differentialamplifier 23 is provided with `a high degree automatic gain control soarranged that the rectified maximum D.-C. output voltage out of thelinear detector y2.6 is always kept constant regardless of fthe 3() mc.input voltage. This is open -to the above-described difficulties andobjections, which ,are eliminated according to the present invention, bythe provision of a variable attenu- .ator 40 in line 15. This variableattenuator is mechanically linked, as indica-"ted by dotted line 44, topiston attenuator 17, as will be shown in detail below, in such mannerthat a linear relationship exists between a moving element of thevariable attenuator 40 and the distance of movement of the pistonattenuator sending coil. lIn this manner it is possible to obtain anoutput voltage from the variable attenuator which is essentiallyconstant, since both devices yare connected to vary in the correct senseso as to accomplish this. One way of doing this would be to put two suchpiston attenuators back-to-back .and drive them in such a manner that ifone decreases its insertion loss the other increases its losscorrespondingly and vice versa. However, it is not necessary to use anactu-al second piston attenuator, which is both expensive and has somedisadvantages for .this purpose, due to its inherently large zeroinsertion loss. In order to have a wide range, the variable attenuatorshould have a minimum loss of only a few tenths of a db. A variablelattenuator which has essentially linear relationship between the outputvoltage expressed in decibels and the movement of a mechanical componentis commercially available; however, it is obvious in the present stateof the art to construct such a device so `as to produce any desiredrelationship between the movement of the mechanical adjusting element ofthe tattenuator and its output. If necessary, an exponential mechanicaldevice can be used with an attenuator not having the desiredcharacteristic inherently.

In the above-described system, let us assurne that we feed an R.F.signal into the system which is at a level of -10 dbm. We now adjust the30 mc. piston attenuator to amplitude balance, with respect to l0 dbm.Since in a practical ease it is required to measure 60 db, thevariableattenuator 4t? should be provided with a range of at least `60 db, or inpractice, somewhat more than the anticipated loss which it is intendedto measure. The gain of the LF. amplifier is then set at some convenientsensitivity point. This can be done automatically as `described in theabove-mentioned patent application. The unit under test is now inserted,which will drop the R.\F. signal input level, to take an extremecondition, from d0 dbm -to perhaps v7O dbm,

representing a 60 db change. This means that the RF. signal voltage isnow a thousand times smaller than it was previously. The LF. signalvoltage, since We have a linear mixer, is also 1,060 times smaller thanbefore.

It the piston attenuator is now adjusted for electrical balance, whichmeans that its attenuation 'is increased by 60 db, then the variableattenuator must also change so that its insertion loss is descreased by60 db, in order to obtain the desired effect. If this is done, the LF.output voltage at this point remainsthe same as before, which is, ofcourse, the objective. We now have the LF. ampliiier operatingessentially at a constant gain with a constant input voltage, andtherefore its frequency response will also be constant.

Referring to FIG: 2, mixer 7 is supplied, as shown in FiG. l, with twoinputs from local oscillator l?. and on line lil respectively. The mixerconsists of the crystal 51, bypass capacitor 52., choke 53, and ltercapacitor 54. The mixer output is supplied on line -13 (line l3nt is theground return) to comparison head i6, which contains decouplingresistor' 55, a Q-adjusting resistor 57, tuning capacitor 53, andreceiving coil 59 of the piston attenuator i7. The sending coil of thepiston attenuator is shown at dii, connected to the 30 mc. source 14.items 56, 57, and 5? together forml the comparison head '16. Thevariable attenuator 4%* is shown as a variable T-type network. Thecrystal current indictor block, shown at 7u in FIG. 2, contains theparallel timed circuit 62, y63, which lis tuned to 30 mc. lIt alsocontains the RF. by-pass capacitor 64 and the low frequen-cy by-passcapacitor 65, as well as the crystal current meter d6. The paralleltuned circuit blocks any 30 rnc. signal going into capacitor 64, 65, ormeter 6d, otherwise the 30 mc. signal would be essentially grounded,since ofi, d5 and e6 together are essentially a short for any othersignal except that to which the circuit is tuned. The LF. differentialamplifier 23 is connected as shown to the output of the variableattenuator di). The manual control element, schematically indicated at7i, is mechanically linked to the receiving coil S9 of the pistonattenuator, as will be shown Ain 4detail in FIG. 3. It will now beapparent that the output from either the LF. signal or the standardsignal will go through the variable attenuator into the LF. differentialampliiier. The above-described system enables improvement in theoperation of the prior art system by allowing the RF. signal or thelocal oscillator generator to have more incidental 'frequency modulationthan was previously allowable, since the LF. ampliiier can now operateat a constant gain.v Previously, 50 kc. was the maximum permissiblefrequency modulation, but with the above system, 5G10 lltc. ofincidental frequency modulation is permissible.

Referring to FIG. 3, the 3G mc. source i4 of the previous iigures ismounted in housing 14o, which is slidably movable on bed 7l byadjustment of rod 72 from right to left, which Imay be done by anysuitable mechanisrn (not shown). Adthough any suitable mechanism may beused, for the purpose of illustration a track 73 is shown mounted on bed7i, and rollers or wheels 74 are shown for slidably support-ing housingMa on tra-ck 3, there being, of course, a similar arrangement on the farside which is not visible. The output of the 30 me. source passesthrough a hollow tube 76, which is very rigidly constructed, and has acentral coaxial conductor 69 (schematically shown in FIG. 2) whereby thetwo elements `form a rigid coaxial conductor leading into the 30 rnc.piston attenuator 17, which is, in er'ect, a hollow block having alongitudinal central aperture therein, in which coil `{t} of the pistonattenuator `is movable relative to coil 59, which is fixed at'the otherend of the longitudinal aperture in block 17. The coaxial conductor 76,o9, bears at its end a sending .coil eil, which moves back and forth inthe aperture in block 17, as housing Illia. is moved. The stationaryreceiving coil S9 leads directly to the comparison head, as also shownan FIG. 2. The 30 mc. LF. signal from the mixer is fed to the comparisonhead on line .l5 (as also shown in FIG. Z) The comparison head elementsshown in FlG. 2 are housed in a casing 65 which is physically lixed tothe bed 7d and therefore remains stationary together with fixed coil 59.The output lead i5 from the comparsion head provides the input to thevariable attenuator 4t?, which is also xed relative to the bed 71 and isof the well-known type having an operating shaft S which is rotated tovary the attenuation of the unit. A pinion 77 is xed to shaft 75, and isrotated by racl; 718,-Whichis in turn iixed to the movable housing 14a,and therefore moves together with movable coil `d0 vof kthe pistonattenuator. lt will thus be apparent that the setting of the variableattenuator is a direct function of the setting of the piston attenuator,which is the required condition. The output of the variable attenuator Y4o is supplied on line 79 to the LF. dilierential amplilier 23. it willbe apparent that various other mechanical arran ements may be employedto secure the same result. A suitable variable attenuator for thepurpose is commercially available in the United States. attenuators havean outputvoltage, expressed in decibels, which is linearly related tothemovement of a rnechanical element. lt will be apparent that a pointerrigidly connected to the shaft 75 can be calibrated in decibels upon auniform circular scale if desired. Although the showing is purelyschematic, it will be understood that ordinarily all oi the conductorsand connectors will be high-frequency circuit elements such as coaxialcables, coaxial connectors, etc.

lt will be apparent that the mechanical arrangement of FlG. 3corresponds to the schematic circuit arrangement of FIG. 2, and shows aphysical means whereby the distance of motion of the movable coil 6l) ofthe piston attenuator is directly proportional to the logarithm of theratio of the two outputs. Therefore, the output voltage appearing atconnector 1'5 will remain constant, which is the desired condition ofope-ration.

It will be :apparent that the embodiments shown are only exemplary andthat various modifications can be made in construction and arrangementwithin the scope of the invention as dened in the appended claims.

We claim: r

l. In an insertion loss test system, a high-frequency signal generator,a standard source of LF., a local oscillator,

a mixer for beating the output of said generator with the output of thelocal oscillator to provide a beat frequency output at the samefrequency as the standard LF. source, and an LF. amplifier; means foralternately supplying the output from said standard source and from saidmixer in counterphase to said LF. amplifier for comparison, said lastmeans including a first variable attenufator in the output circuit ofthe standard source, said attenuator having a movable element adjustableto vary the attenuation, a comparison head having input terminalsrespectively supplied by the output of said first attenuator and Iby themixer output, and having also an output terminal, a second variableattenuator connected between said output terminal and said I.F.amplifier, said second attenuator having a'V` movable element forvarying the attenuation thereof, VandV linkage means between the movableelements of the iirst and second attenuators for varying the attenuationof said second attenuator in accordance with the adjustment of A saidfirst attenuator.

2. The invention according to claim 1, said first attenuator being apiston attenuator Vand its movable element being linearly movable toadjust its attenuation.

3. The invention according to claim4 2, said second attenuator having arelationship between the amount'of its motion and its output which issubstantially the same as that of the first attenuator, the linkagebetween the two being such that they vary in opposite senses so as tomaintain `a constant input to the LF. amplifier as said irst attenua-These i i tor is adjusted to a condition of balance with the mixeroutput.

4. The invention according to claim 3, the movable element of saidsecond attenuator being a rotary element, said linkage means between thetwo attenuators being a mechanical linkage for transferring the linearmotion of the piston attenuator to rotary motion for adjusting saidrotary element.

5. In an insertion loss test system of the type comprising ahigh-frequency signal Igenerator, `a standard source of LF., a localoscillator, a mixer for beating the outputs of the local oscillator andof said generator to provide a beat frequency output at the samefrequency as the standard LF. source, and an LF. amplifier;

(a) means for alternately supplying the output from said standard sourceand from said mixer in counterphase to said LF. :amplifier forcomparison,

(b) said last means including a first variable attenuator in the outputcircuit of the standard source, said attenuator having a movable elementadjustable to vary the attenuation,

(c) a comparison head having input terminals respectively supplied bythe output of said irst attenuator and by the mixer output, and havingalso an output terminal,

(di) va secon-d variable attenuator connected between said outputterminal and said LF. amplifier,

(e) said, second attenuator having a movable element for varying theattenuation thereof,

(f) ylinkage means between the movable elements of the iirst and secondattenuators for varying the attenuation of said second attenulator inaccordance with the adjustment of said first attenuator, but in anopposite sense, so las to maintain a constant output to the I F.amplifier as said -irst attenuator is adjusted to -a condition ofbalance with the mixer output.

References Cited in the file of this patent UNITED STATES PATENTS2,419,527 Barte-link Apr. 29, 1947 2,424,833 Korman I uly 29, 19472,499,755 Hunt Mar. 7, 1950 2,587,493 Loughlin Feb. 26, 1952 3,029,3364Ferrar Apr. l0, 1962

1. IN AN INSERTION LOSS TEST SYSTEM, A HIGH-FREQUENCY SIGNAL GENERATOR,A STANDARD SOURCE OF I.F., A LOCAL OSCILLATOR, A MIXER FOR BEATING THEOUTPUT OF SAID GENERATOR WITH THE OUTPUT OF THE LOCAL OSCILLATOR TOPROVIDE A BEAT FREQUENCY OUTPUT AT THE SAME FREQUENCY AS THE STANDARDI.F. SOURCE, AND AN I.F. AMPLIFIER; MEANS FOR ALTERNATELY SUPPLYING THEOUTPUT FROM SAID STANDARD SOURCE AND FROM SAID MIXER IN COUNTERPHASE TOSAID I.F. AMPLIFIER FOR COMPARISON, SAID LAST MEANS INCLUDING A FIRSTVARIABLE ATTENUATOR IN THE OUTPUT CIRCUIT OF THE STANDARD SOURCE, SAIDATTENUATOR HAVING A MOVABLE ELEMENT ADJUSTABLE TO VARY THE ATTENUATION,A COMPARISON HEAD HAVING INPUT TERMINALS RESPECTIVELY SUPPLIED BY THEOUTPUT OF SAID FIRST ATTENUATOR AND BY THE MIXER OUTPUT, AND HAVING ALSOAN OUTPUT TERMINAL, A SECOND VARIABLE ATTENUATOR CONNECTED BETWEEN SAIDOUTPUT TERMINAL AND SAID I.F. AMPLIFIER, SAID SECOND ATTENUATOR HAVING AMOVABLE ELEMENT FOR VARYING THE ATTENUATION THEREOF, AND LINKAGE MEANSBETWEEN THE MOVABLE ELEMENTS OF THE FIRST AND SECOND ATTENUATORS FORVARYING THE ATTENUATION OF SAID SECOND ATTENUATOR IN ACCORDANCE WITH THEADJUSTMENT OF SAID FIRST ATTENUATOR.